The caloric test is a specialized diagnostic procedure used to evaluate the function of the vestibular system in the inner ear, which maintains balance and spatial orientation. It is typically performed as part of a larger battery of assessments called Videonystagmography (VNG) or Electronystagmography (ENG). By using thermal stimulation, the procedure allows clinicians to examine the balance function of each inner ear separately, helping to identify and localize potential damage contributing to dizziness or vertigo.
Purpose of the Caloric Test
Doctors order this test to investigate persistent dizziness, vertigo, or unexplained balance difficulties. The caloric test specifically assesses the integrity of the horizontal semicircular canal and the superior branch of the vestibular nerve. This targeted assessment is achieved by stimulating the inner ear’s balance organ using temperature changes.
The test helps determine if a balance problem originates in the peripheral system (the inner ear) or the central system (brainstem and cerebellum). Since it stimulates each ear independently, the caloric test is effective for detecting unilateral weakness, a common indicator of inner ear damage like vestibular neuritis. Identifying the location of the problem is a crucial step in developing an effective treatment plan.
Step-by-Step Procedure
The caloric test begins with the patient lying on their back with their head elevated approximately 30 degrees. This positioning aligns the horizontal semicircular canal into the vertical plane, making it sensitive to temperature change. The patient wears specialized goggles, often containing infrared cameras (VNG), to continuously record involuntary eye movements in darkness, which removes visual fixation.
The procedure involves introducing thermal stimuli—warm or cool air or water—into the external ear canal, one ear at a time. Standard temperatures are typically 44°C for warm stimulation and 30°C for cool stimulation, lasting about 30 to 60 seconds each. This temperature difference creates a convection current within the endolymph, the fluid inside the semicircular canal.
The movement of the endolymph fluid mimics the flow that normally occurs during a head turn, effectively “tricking” the inner ear into thinking the head is moving. This artificial stimulation triggers the vestibulo-ocular reflex, resulting in a predictable, involuntary eye oscillation called nystagmus. Warm stimulation causes nystagmus to beat toward the stimulated ear, while cool stimulation causes it to beat away. After the first ear is tested, there is a waiting period of several minutes to allow the induced nystagmus to resolve before testing the second ear.
Interpreting the Test Results
Clinicians analyze the recorded eye movements to diagnose the balance disorder. The primary metric measured is the maximum slow-phase velocity (SPV) of the nystagmus, which reflects the speed of the eye movement and the strength of the inner ear’s response. A normal result shows a symmetrical, robust response from both ears, confirming that the balance organs are functioning equally.
An abnormal finding is often categorized as Unilateral Weakness (UW), a significant difference in the response strength between the two ears. UW is calculated by comparing the total response from one ear against the total response from both. A UW value exceeding 25% to 40% typically indicates a peripheral lesion on the weaker side, such as damage to the labyrinth or vestibular nerve. Unilateral Weakness is considered the best indicator of peripheral vestibular system dysfunction.
Another metric analyzed is Directional Preponderance (DP), which indicates if the nystagmus is consistently stronger when beating in one direction, regardless of the ear stimulated. A significant DP may be caused by spontaneous nystagmus, indicating peripheral or central nervous system involvement. Results are also assessed for Bilateral Weakness, where both ears show a severely reduced response, suggesting a widespread issue affecting both inner ears, possibly due to medications or systemic diseases.
Preparing for the Test and Recovery
To ensure accurate results, patients are instructed to avoid central nervous system suppressants for up to 48 hours before the appointment. These include vertigo suppressants, sedatives, tranquilizers, and certain antihistamines, as they can artificially weaken the inner ear’s response. Patients should also avoid alcohol and caffeine for 24 hours and should not eat a heavy meal immediately preceding the test to minimize the risk of nausea.
The thermal stimulation is intended to induce a temporary feeling of vertigo and dizziness, which is a normal part of the procedure. Patients may also experience mild nausea or lightheadedness because the brain receives conflicting signals about movement. These effects are short-lived and typically resolve within a few minutes after the irrigation is complete. Due to the temporary dizziness, patients must arrange for someone else to drive them home after the appointment.